How big the fuse should be?

The combined value of 140 amps is the correct assumption. Now what you are hearing can be taken a number of ways. In 99% of the cases, the fuse has nothing to do with the amplifiers, remember though there are exceptions and we'll get to that. Even if you had a 140 amp fuse up front, under what circumstances are you going to be drawing this much current and for what continuous period of time. Most fuses are rated to pass much more current safely for breif moments, it is when they are asked to pass a large amount of current for a sustained time that they will blow. Now with the exception still outstanding, if you are the kind of person who is going to play the system long and hard and annoy just about everyone, chances are that a large value fuse will be necessary, if you want to listen to the system at a moderate yet sustained level a large to mid value fuse would be used. If you listen to a system that has 140amps of fusing at the amplifiers and want to listen to this system and don't plan on turning it up, you will be drawn and quartered.

The only problem with a fuse too small is that it may blow if it asked to do a job it was not designed for. If the fuse is too big, so what, the amplifiers will only draw what they need through it. Like I said with the exception still outstanding, the purpose of this main fuse is to protect the vehicle and it's contents in the event of a short circuit, most commonly cause by the power wire grounding out, might be do to chaffing or a nasty car accident.

Now for the exception, some amplifiers, like the older Rockford, Precision, Lineap Power, Orionand even Alphasonik did not have built in fuses. The manufacturer recommended to use x size fuse at the battery. This still meant that it could be fused at the back by the amp and a single large fuse at the battery, but this is the exception to above. Hope this helps out.